Concept on positioning of Columns
Question 1:
Comment on the layout of the framing plan concerning earthquake resistance in the two horizontal directions X or Y (dots are columns, lines depict beams). Look into aspects of symmetry, redundancy and bi-directional stiffness and ductility.
Answer:-
Symmetry:-
It is observed that the distribution of mass,stiffness is across the plan has good symmetrical aspects.
This tends to reduce the horizontal displacements in both of the two horizontal displacements in both of the two Orthogonal Horizontal Directions.
It also has a very good uniformity also.
(i). Symmetry in X - Direction:
If We split the above plan in along X - Direction, its seems mirrored in plan.
Hence, it is symmetrical in X - Direction.
(ii). Symmetry in Y - Direction:
If We split the above plan in along Y - Direction, its seems mirrored in plan.
Hence, it is symmetrical in Y - Direction.
Result:-
We conclude that, the above plan is symmetrical in both direction.
Redundancy:-
There are missing structural elements in the Periphery of the plan.This causes the uneven distribution of earthquake action effects in non linear range across the plan.
The energy dissipation across the structure is very low which results in non -reliable structure.
(i). Redundancy in X - Direction:
In the above given plan there are multiple dots and the lines.
Dots represents the columns and the line represents the beams.
We can observe that the columns in the X-directions are more and evenly spread in the middle portion of the plan.
Hence, we can say that the structure is redundant in the X-direction.
The colums arrangement in the top and the bottom rows are very less when compared to the middle portion of the plan.
But these columns are evenly spreaded, so we can say that the structure in the X-direction is redundant in nature.
(ii). Redundancy in Y - Direction:
We can observe that, the column arrangements in Y - Direction are quite lesser when compared to the column arrangements in X - Direction.
But the columns in the Y-directions are evenly spread but they are lesser in number.
We can say that the structure is reduntant in Y - Direction. but compare with X - Direction it is lesser in redundancy.
Result:-
After checking the redundancy in the X and Y direction we can conclude that the structure is more redundant in the X-direction than the Y-direction.
Bi-Directional Stiffness:-
The building plan may not be able to resist the horizontal seismic actions in the shorter direction (Y-axis) and also the resistance and stiffness in the shorter direction (Y-axis) is less when compared to longer direction (X-axis).
(i). Bi-Directional Stiffness along X - Direction:
As we can see in the above given plan we observe that the columns in the X-directions are connected in a straight line with the beams in the continuous manner.
Thats why we can say that the loads can be carried out and the loads can be transferred easily in the horizontal direction i.e, X-direction.
After this study we can conclude that the frame is stronger in the bi-lateral stiffness in the X-direction.
(ii). Bi-Directional Stiffness along Y - Direction:
If we see the above given plan thoroughly than we can see that the columns in the Y-direction connected in a straight line, but as we can see in the above given image that the beams are not connecting all the columns properly.
As we can see that there are empty space after the each one beam.
Therefore the loads can not be transferred easily in the Y-direction.
Therefore we can conclude that the above given frame is weak in the bi-lateral stiffness in the Y-direction.
Result:-
After checking the bi-lateral stiffness in both the direction i.e, X and Y direction we can conclude that the frame is stronger in the X-direction according to the bi-lateral stiffness that the Y-direction.
Ductility:-
The building may not be able resist the elastic earthquake forces in shorter direction (Y-axis).So it may result in inelastic Response.
The reduction in design force in the shorter direction (Y-axis) is to be compensated by Ductility.
(i). Ductility along X - Direction:
As we can see in the above given plan the beams are connecting all the columns in the X-direction and the stiffness is more in the X-direction because the number of columns are more the X-direction.
So we can conclude that the frame is more ductile in the X-direction.
(ii). Ductility along Y - Direction:
As we can see in the above given plan that the beams are not connected to each and every column of this frame in the Y-direction.
And the stiffness of this frame in the Y-direction is also less because of the lesser number of the beams in the Y-direction.
So we can conclude that the frame is not ductile in the Y-direction.
Result:-
So if we compare the ductility in both the direction i.e, X and Y direction then we can say that the frame in the X-direction is more ductile and the frame in the Y-direction is not ductile due to the lesser number of beams in the Y-direction.
Question 2:
What are the possible sources of irregularity in the building shown here? Explain how, as an engineer, each of these irregularities should be dealt with.
Answer:-
The possible type of irregularities for the given structure are:
Soft storey irregularity:
- Stiff and strong upper floor due to masonary fill
- The column in one storey is longer than the above.
Some provisions to avoid the irregulrities:
The building is torsionally irregular.So the building framing must be revised.
The mass irregulrity is avoided by providing light weight concrete/ Low weight masonary or by providing structure with more wood materials.
The structure should be built in a well symmmetric manner.
The columns which are provided on the structure should be of same size and height through out the building.
The weight of the building from ground floor should decrease towards top.
The possible sources of irregularity in the building are listed and discussed below:
Failure at the middle of the building due to the soft-storey.
Vertical irregularity.
Slopy base.
(i) Soft storey:-
A soft story building is a multi-story building in which one or more floors have windows, wide doors, large unobstructed commercial spaces, or other openings in places where a shear wall would normally be required for stability as a matter of earthquake engineering design. A typical soft story building is an apartment building of three or more stories located over a ground level with large openings, such as a parking garage or series of retail businesses with large windows.
As we can see in the above given image that this building at the 6th floor has a soft storey because it has uncovered opening.
As we can see that there is no partition wall or no occupancy or any other shear wall or any other type of support elements etc nothing is there to support the upper blocks of the building during the earthquake and this will also cause plastic hinge at the colums. According to the reference IS 1983-2016 Figure 4A, this has to be checked and we have to design the building accordingly. So that there will be no mistake in designing the buiding.
Below are the images showing the stiffness irregularities(soft storey):
(ii) Vertical irregularities:-
During an earthquake, failure of structure starts at points of weakness. This weakness arises due to discontinuity in mass, stiffness and geometry of structure. The structures having this discontinuity are termed as Irregular structures. The behaviour of a building during an earthquake depends on several factors such as stiffness, adequate lateral strength, ductility and configuration. The buildings with regular geometry and uniformly distributed mass and stiffness in plan as well as in elevation suffer much less damage compared to irregular configurations. Regular buildings have uniformly distributed mass, stiffness, strength and structural form. When one or more of these properties is non- uniformly distributed, either individually or in combination with other properties in any direction, the structure is referred to as being irregular. Irregular structures contribute a large portion of urban infrastructure. Vertical irregularities are one of the major reasons of failures of structures during earthquakes.
If we observe the above given plan carefully then we will be able to see that the buiding plan area is bigger in the bottom section and the building plan area keeps reducing as it goes up. In short it means that the building plan area reduces with the increase in the height as we can see clearly in the above image.
The area in the green rectangle has bigger plan area than that of the area that are in the yellow rectangle.
The building offset has to be checked out with the reference of IS 1893-2016 table no.6 serial no (iii). The image regarding this reference is given below:
(iii) Slopy base:-
If we observe properly then we can see that at the right hand side of the image there is slopinf ground and this building is constructed at that sloping area.
Which means that the there are risks that this building may collapse during the earthquake.
And this will lead to the variation in the stiffness as we can conclude properly.
So according to the IS 1893-2016 FIG 5E we have some standards in order to design a building insuch slopy areas and if we follow these standards then there is high possibility that we can save the structure from collapse during the earthquake attack.
Question 3:
A 4-storey hotel building has an open ground floor for the restaurant. Story 2 to 4 have one row of rooms along each long side in plan, separated by a corridor. The two short sides of the perimeter are fully in-filled in all storeys, except for certain openings at the ends of the corridor at storeys 2 to 4 and along the right-hand side of the ground floor. There is a staircase near the upper left-hand corner, with straight flights between landings at floor levels and in-between floors. Cross-section dimensions are written next to the member no. in meters.
Comment on the features of the structural design and of the layout of infills which are important for earthquake resistance and seismic performance. How do they relate to the almost full collapse of this building (the extreme left-hand bay with the staircase survived, as well as one long-side façade in the rear end, and the frame along the right-hand side in plan).
Answer:
(a) Open ground floor:-
As it is already mentioned in the question that the hotel building has an open ground floor for the restaurant, it means that the restaurant has a soft storey mechanism acting on the building that cause the damage to the building. And this is one of the reasons why this restaurant building collapse.
(b) Rooms along two bay separated by an open space:-
The bay that are filled with the rooms will have higher mass than the middle bay whic is opened for the corridor.
So this will cause mass mass irregularity along the height uniformity.
As we can see at the above image that the upper portion and the bottom portion is for the rooms and because of that rooms these portions have higher mass because of the various furniture load,live loads etc.
And the middle portion they left for the corridor as we can see in the above given image.
(c) Two short sides coveres with the infill walls:-
As we can see that the two shorter sides of the perimeter are not in filled in all the storeys,except for the certain openings at the ends of the corridor at the 2nd and the 4th floor and along the right-hand side of the ground floor.
This means that the building at the left and the right of the given building is more stiffer during the earthquake.
And they have built non structural wall.
(d) Staircase portion at the left top corner:-
As we can see that the staircase and the lift portion of the building is not affected with the seismic force. These are the only elements that are not affected during the earthquake force.
This is because the usually the staircase and the lift portion are more stiff than the other members of the building. It m eans it does not bend easily with the external forces and that is the reason that the staircase and the lift is intact in the polsition with much harm.
As we can see in the image only the staircase part is intact because of the concrete elements.
So the staircase is the only intact portion which contains more stiffness in the given structure or hotel.
(e) No frame connection along the Y-direction:-
As we can see in the above image that there are no structural beams that are provided along the Y-direction between the columns.
This means that the building is more vulnerable during the earthquake because of the no frame connection along the Y-direction.
Thats why we have to be more careful while designing the structures because a little mistake can lead to a very serious problem.
(f) Beams connected along column minor axis:-
As we can see in the above image that the columns have the longer direction in the Y-direction.
But there is no direct connection or frame in the Y-direction which may connects the beam in the Y-direction.
There are columns that are connected in the X-direction but the large side of the column is in the Y-direction which means that they are more stiffer in that direction whic can attract more lesser lateral force and this is one of the reason for the collpase of this building.
(g) Variation in column sizes:-
As we can see in the above given image that the sizes of the columns are different.
We can say that the size of the interior column is big in size and the size of the exterior column is smaller as wecan see in the above image.
And because of the difference in the size of the columns the structure collapse because it may suffered heavy out of plan shear.